Dry air is heavier than moist air because the average molecular mass of the molecules that make up dry air is greater than the molecular mass of water vapor (H₂O) that replaces some of those molecules in moist air.
Here's a breakdown:
-
Composition of Air: Air is primarily composed of nitrogen (N₂) and oxygen (O₂). Other gases like argon (Ar) are present in smaller amounts. Dry air consists almost entirely of these gases.
-
Molecular Masses:
- Nitrogen (N₂): Approximately 28 atomic mass units (amu)
- Oxygen (O₂): Approximately 32 amu
- Water (H₂O): Approximately 18 amu
-
The Key Concept: Replacement: When water vapor (H₂O) is added to air (creating moist air), it displaces other air molecules (primarily N₂ and O₂).
-
Lower Molecular Mass of Water Vapor: Since water vapor has a significantly lower molecular mass (18 amu) compared to nitrogen (28 amu) and oxygen (32 amu), replacing nitrogen and oxygen molecules with water molecules reduces the average molecular mass of the air.
-
Density and Weight: At the same temperature and pressure, air with a lower average molecular mass is less dense, and therefore, lighter. Essentially, a "packet" of moist air will contain fewer heavy nitrogen and oxygen molecules and more lighter water vapor molecules, resulting in a lower overall weight.
In Summary: Dry air contains heavier nitrogen and oxygen molecules. Moist air contains a mix of these molecules along with lighter water vapor molecules. Because water vapor is lighter than the nitrogen and oxygen it displaces, moist air is lighter than dry air. This isn't intuitive because we often associate "humidity" with a feeling of heaviness, but this is a sensation related to the difficulty of sweat evaporating in humid conditions, not the actual weight of the air.
